1. Field of the Invention
The present invention relates to a charge/discharge current detection circuit that detects a charge current when charging a battery (chargeable secondary battery, storage battery and the like) and its discharge current, and a variable resistor that is preferentially used as a component of the charge/discharge current detection circuit.
2. Description of the Related Art
Conventionally, a circuit shown in FIG. 6, for example, is known as a charge/discharge current detection circuit of this type.
As shown in FIG. 6, the charge/discharge current detection circuit is formed from components such as a detection resistance Rs, a charge current monitor amplifier 1, a discharge current monitor amplifier 2, a buffer amplifier 3, and an offset voltage adjusting circuit 4.
The detection resistance Rs is used to detect a charge current to a battery, or to detect a discharge current from the battery to a load. The charge current monitor amplifier 1 is an amplifier circuit that is composed of an operation amplifier OP1 and resistances RC1 and RC2. The discharge current monitor amplifier 2 is an amplifier circuit that is composed of an operation amplifier OP2 and resistances RD1 and RD2. An output from either the charge current monitor amplifier 1 or the discharge current monitor amplifier 2 can be retrieved by selectively switching switches SW1 and SW2.
The buffer amplifier 3 is composed of an operation amplifier OP3 and resistances R1-R4. The offset voltage adjusting circuit 4 can set a desired offset voltage by an externally provided instruction at the time of adjusting the offset voltage.
Next, operations of the charge/discharge current detection circuit having the structure described above are described with reference to FIG. 6.
First, the description is made as to a case when the switch SW1 is closed, and the charge current monitor amplifier 1 detects a charge current to a battery. In this case, because the charge current flows through the detection resistance Rs in a direction indicated in FIG. 6 such that a voltage is generated, a voltage drop is applied to a (+) input terminal of the operation amplifier OP1, and the applied voltage is amplified and outputted.
On the other hand, when the switch SW2 is closed, and the discharge current monitor amplifier 2 detects a discharge current from the battery to a load, the discharge current flows through the detection resistance Rs in a direction indicated in FIG. 6 such that a voltage is generated, and a voltage drop is applied to a (+) input terminal of the operation amplifier OP2, and the applied voltage is amplified and outputted.
Due to the fact that there are different offset voltages present respectively for the charge current monitor amplifier 1 and the discharge current monitor amplifier 2 in the conventional charge/discharge current detection circuit, the offset voltages need to be set independently from one another by the offset voltage adjusting circuit 4, and the setting operation is complicated, which is inconvenient.
Also, due to the fact that the two amplifiers, the charge current monitor amplifier 1 and the discharge current monitor amplifier 2, are used, the gain and other characteristic factors (linearity error or the like) at the time of detecting a charge current are different from those at the time of detecting a discharge current, such that the charge current and the discharge current cannot be correctly compared, which is also inconvenient.
To solve the inconveniences described above, the present inventors have made diligent research and studies, and as a result, invented a novel charge/discharge current detection circuit.
In the mean time, the novel charge/discharge current detection circuit needed a variable resistor. However, it became clear that the use of a conventional variable resistor, for example, the one shown in FIG. 7, would cause the following problems. The problems will be described below.
In the conventional variable resistor shown in FIG. 7, resistances R11-R16 are serially connected to one another, and MOS transistors Q1-Q6 are connected as switches to both ends of the respective resistances R11-R16. The MOS transistors Q1-Q6 are controlled to be turned on and off to set a desired resistance value.
Here, resistance values of the resistances R11-R16 have a relation in which, when the resistance value of the resistance R1 is R [Ω], the resistance values of the resistances R12-R16 are 2×R, 4×R, 8×R, . . . , respectively.
However, since the conventional variable resistor uses MOS transistors that are turned on and off, their ON resistance becomes problematical. For example in FIG. 7, when a state in which only the MOS transistor Q1 is in an ON state (a state of the maximum resistance value) changes drastically to a state in which the MOS transistors Q2-Q6 are in an ON state (a state of the minimum resistance value), it assumes a state in which the ON resistances of the respective MOS transistors Q2-Q6 are serially connected to the resistance R11, and the sum of the ON resistances becomes an error, which is problematical.
For this reason, conventionally, the resistance value of the resistance R11 needs to be made large, or the transistor size needs to be made larger to reduce the ON resistances of the MOS transistors in order to solve the problems described above.
Accordingly, in view of the aspects discussed above, it is a first advantage of the present invention to provide a charge/discharge current detection circuit in which the detection of charge current and discharge current can be conducted under the same operational conditions, characteristic factors of an amplifier circuit have similar influences upon detecting charge current and discharge current such that the charge current and discharge current can be correctly compared, and offsets can be readily adjusted.
Also, it is a second advantage of the present invention to provide a variable resistor that can adjust the resistance valve with a high degree of accuracy and is suitable for, for example, a charge/discharge current detection circuit or the like.